monotremes, marsupials, placentals

[caz]

hi. i'm a first-time poster on here and i'd like to point out straight away that i don't have a science background and am a layman who is fascinated by natural history.

 

was wondering whether anyone could tell me what the best current thinking is on how and when the mammals split into the three groups we have today - the placentals, the marsupials and the monotremes.

 

in particular, do the marsupials date back further than the placentals and, if so, did the placentals descend directly from them or, more likely, diverge later from a common ancestor.

 

i asked a similar question on another forum and a couple of guys on there were quite helpful in telling me that the fossil evidence is ambiguous and, basically, no one knows.

 

so it would seem that we know more about the dinosaurs than about our closer mammalian cousins.

 

common sense tells me that the monotremes (platypus and echidna) are the most ancient of the groups that are still around. they lay eggs and have other features that place them closer to birds and reptiles.

 

the same thing tells me that the marsupials pre-date the placentals and split from a common ancestor before mammals had the ability to develop a proper placenta.

 

both the monotremes and marsupials suckle young that seem almost embryonic and it's interesting that the female echidna (the forgotten monotreme) develops a simple pouch in which to lay her egg.

 

The "puggle" (as i believe a little echidna is known) is not much bigger than a jellybean when hatched and is carried around in her pouch for about three months.

 

I do think marsupials are a more ancient lineage than placentals, but, the fossil record seems to be ambiguous.

 

any thoughts on this or good, simple further reading would be appreciated.

[granpa]

do monotremes produce milk? what about marsupials?

 

placentals (which lack a cloaca) require much more water than marsupials. maybe that has something to do with their split.

Edited January 6, 2009 by granpa

[caz]

do monotremes produce milk? what about marsupials? .

 

absolutely. both do, but the monotremes had not yet evolved nipples and that is another thing that suggests how ancient their line is.

[granpa]

I understand that the young marsupials latch onto a part of the wall of the pouch which then expands in their mouth making it impossible for them to release it. no doubt this is how nipples evolved. that would imply that placentals did evolve from marsupials.

[caz]

I understand that the young marsupials latch onto a part of the wall of the pouch which then expands in their mouth making it impossible for them to release it. no doubt this is how nipples evolved. that would imply that placentals did evolve from marsupials.

 

as i understand it, marsupials such as kanagaroos have nipples but monotremes (platypus and echidna) do not.

 

so, did the placentals descended directly from marsupials or from a monotreme-like common ancestor?

 

current thinking seems to be that they developed at about the same time in different parts of the world. but, to me, the marsupials seem more ancient.

Edited January 6, 2009 by caz

[granpa]

marsupials could have evolved independently in the arctic and antarctic. arctic marsupials could then have evolved into placentals. the big question to me is why the arctic and antarctic took such different paths.

[caz]

marsupials could have evolved independently in the arctic and antarctic. arctic marsupials could then have evolved into placentals. the big question to me is why the arctic and antarctic took such different paths.

 

it's complicated. the earliest marsupial fossil (125 million year old) was found in china and the ones found in australia are much younger.

 

others have been found in the americas and the only mammal fossil found in the antarctic was of a marsupial.

 

http://www.sciteclibrary.ru/eng/catalog/pages/6763.html

[Mokele]

Basically, all mammals evolved from a group of reptiles called the synapsids. Over time, the synapsids changed form, and most of the old forms died out. At some point after milk evolved, but before live birth, the ancestors of the modern monotremes split off from the ancestors of all other mammals, and it's worth noting that modern monotremes are actually very different from their ancestors (which resembled small shrews or rats). At some point after the ancestor of other mammals split from monotremes, that ancestor split into the ancestors of marsupials and placentals, both of which have changed greatly.

 

It should be noted that there is no support, fossil or molecular, for multiple origins of any of these modern groups. The present distribution of marsupials is due to continental drift - when they evolved, Australia, Antarctica, and South America were all joined, and separate from other continents. Over time, they split, Antarctica became cold, and the Americas joined, at which point some marsupials moved north.

 

Mokele

[granpa]

milk evolved before live birth but after the evolution of pouches. pouches (which are presumably adaptations to cold) could conveivably have evolved independently in the northern and southern hemispheres.

[Mokele]

Could, but where's the evidence? In the absence of the evidence to the contrary, it's most likely the pouch evolved only once.

[granpa]

you havent been following the discussion. I pointed out that nipples apparently evolved from the area of the pouch that the joey attaches to. he said that current thinking is that placentals and marsupials evolved independently. so I said that pouches could conceivably have evolved separately in the northern and sounthern hemispheres.

[Mokele]

I know, but I see no reason to assume that nipples evolved after pouches. If nipples evolved before the split between marsupials and placentals and before pouches, but after the split with modern monotremes, then there would be no need to posit more than one origin of either.

[granpa]

nipples apparently evolved from the area of the pouch that the joey attaches to.

 

that isnt a reason to suspect that nipples evolved after pouches did?

[Mokele]

It would be, if that were actually true. Is there any evidence at all to support that assertion?

[granpa]

what assertion? nobodyknows for certain where they came from. thats why I said 'apparently' (it appears to me). if you dont believe that then fine. thats your opinion and its noted. this is my last post in this thread.

Edited January 9, 2009 by granpa

[Mokele]

If you're postulating the origins of traits, it's generally accepted that, without evidence to the contrary, you should prefer a hypothesized pattern which minimizes the number of changes. See Maximum parsimony.

[granpa]

obviously. that is my thinking too. but I was responding to this from post 5

 

current thinking seems to be that they developed at about the same time in different parts of the world. but, to me, the marsupials seem more ancient.

 

if your not going to follow the discussion then please dont comment on it.

I have nothing further to say.

[Mokele]

Oh I followed it. It was just mostly garbage.

[caz]

I know, but I see no reason to assume that nipples evolved after pouches. If nipples evolved before the split between marsupials and placentals and before pouches, but after the split with modern monotremes, then there would be no need to posit more than one origin of either.

 

 

echidnas have pouches but not nipples. so in their case pouches came first.

 

if the pouch did evolve only once as you say, it would suggest that marsupials descended from something like the pouched monotreme echidna.

Edited January 9, 2009 by caz

[Mokele]

Entirely possible, yes. The problem is that pouches and nipples don't fossilize - the closest thing we have are the epipubic bones, but that's confounded by the fact that they also serve a locomotor function.

[caz]

Entirely possible, yes. The problem is that pouches and nipples don't fossilize - the closest thing we have are the epipubic bones, but that's confounded by the fact that they also serve a locomotor function.

 

i also find it interesting that the marsupial embryo reportedly has a sort of a vestigial egg shell and an egg-tooth ... both fully functional features of baby monotremes.

 

if that's true then, reproductively at least, it's not that big an evolutionary leap between an echidna (which lays its tiny eggs in its pouch) and a typical marsupial (which discards the egg shell within its body and guides it's tiny young into the pouch).

 

monotremes and marsupials also share epipubic bones, which i've done a bit of reading up on.

 

i found it interesting that the extinct thylacine (which had a pouch) had only the vestiges of them and certain pouchless marsupials still have the real deal. go figure!

 

perhaps they are more related to the locomotor function.

Edited January 12, 2009 by caz